Accurate measurement devices are needed by modern manufacturing facilities where the machining accuracy approaches several microns per meter so that the accuracy of the measuring devices for inspection and control must be in the sub-micron range. Among the existing measuring systems only interferometers can provide such accuracies over extended ranges (up to and beyond 1 meter). Other systems suffer either from limited accuracy or from limited range. Most commercial interferometers, which are capable of measuring ranges up to and above 10 meters and of accuracies of 0.1 .mu.m/m or better, are based on stabilized lasers and are too delicate or too expensive to be used widely on the factory floor or be incorporated in systems for closed-loop motion control. Interferometers based on non-stabilized lasers, on the other hand, have a very limited measuring range.
The main obstacle to the use of non-stabilized lasers in displacement measuring interferometry (and in unequal-path interferometers in general) is their limited coherence length and, hence, limited measuring range. This is caused by simultaneous presence of several resonating cavity modes in laser emission (mode competition). Each of the modes forms its own interference fringe system, and these systems are shifted with respect to each other in accordance with the frequency difference between the modes and with the measured path length. The optical signal arriving at the detection system of an interferometer carries the interference fringe systems associated with each mode and, thus, can be regarded as "encoded" with the optical signals of the individual modes. The interference systems of different modes overlap incoherently because of a very large frequency difference between the modes compared with the frequency response of the detectors. Therefore, the fringe picture observed by the detection system is a sum of the fringe intensities of all the modes present, and the "encoded" information pertaining to the individual modes is lost.
Consider, for clarity, a short-tube laser which emits most of the time in two modes. The frequency spacing of the modes is c/2L, where L is the tube length of the laser. (This is illustrated in FIG. 1a, where the P.sub.2 mode is, typically, below the gain threshold and does not participate in emission). If the optical path difference in the interferometer is equal to the tube length, then the fringe pictures of the two modes at detection are exactly in antiphase. This will be observed as a fringe picture of the stronger mode with fringe contrast reduced by the contamination by the second mode. If, in addition, the intensities of the modes are equal (FIG. 1b), then the loss of contrast will be complete. The intensities of the modes depend on their relative positions under the Doppler profile of the laser emission line (FIG. 1) and, in turn, on the laser tube length, and they fluctuate with inevitable fluctuations of the latter caused mainly by temperature variations. Thus, when the optical path difference approaches the tube length (i.e., when the displacement to be measured approaches its half) and the fringe systems of the two modes are in antiphase, partial loss of contrast will be observed and the contrast will fluctuate with time between almost 100% in the case of FIG. 1a and zero in the case of FIG. 1b. The coherence length of such a laser is equal, therefore, to its tube length.
In general, the coherence length of a multimode non-stabilized laser is 2L/m, where m is the number of modes, and in practice measuring ranges for, say, HeNe lasers, are only of the order 100 mm.